Mechanisms will be analyzed by which adenovirus E1B-55K in vitro indicated that a cellular co-repressor(s) is required to inhibit a step in basal transcription specifically from promoters with p53-binding sites. We propose to purify this co-repressor and analyze the mechanism by which it represses transcription. In a p53-minus cell line, the principal defect in the replication of the E1B- 55K null mutant d11520 at 32 degrees is failure to efficiently translate viral late mRNAs. Remarkably, this defect is largely complemented by incubation at 39 degrees. This observation suggests that induction of the heat-shock stress response can largely substitute for the E1B-55K late function. This possibility will be tested by determining if chemical agents that induce the stress response at 32 degrees also relieve the requirement for E1B-55K. During the late phase of infection by wtAd5, host cell mRNA translation is inhibited by the dephosphorylation of the translation initiation factor eIF-4E, the cap binding complex. An E1B- 55K mutant has been reported to be defective in inducing this eIF-4E dephosphorylation. Since heat shock also indues eIF-4E dephosphorylation and elevated temperature largely relieves the requirement for E1B-55K, we propose studies to test the model that the dephosphorylation of eIF-4E is required for the efficient translation of viral late mRNAs. We will also test the model that dephosphorylation of eIF-4E indirectly causes the late phase inhibition of cellular mRNA nucleocytoplasmic transport by preventing the release of shuttling hnRNP proteins from newly transported mRNAs. These studies may provide a simple means for predicting which tumor cells would be effective hosts for the replication of E1B-55K mutants and, therefore, might be candidates for therapy by infection with an E1B-55K mutant.